Who was Joe Gerk?

A friend of my Mum's has told me that Joe Gerk's is slang for Walton Prison and asked if I know who Joe Gerk was. I'd never heard the term before but it seems that it is a Scouse term for Walton. Does anyone know who Joe Gerk was or is it perhaps rhyming slang? My curiosity has been piqued now! Thanks.

I had a look at that Oudeis and it seems:
gerk butty: slice of plain, uncoated bread
So maybe Joe Gerk's is where the prisoners would eat dry bread and water (in the old days!). The mystery thickens!
thanks for your help anyway.